1. Field of the Invention
This invention relates to a method for synthesizing amorphous silicon nitride. More particularly, it is concerned with a method for efficiency synthesizing and recovering amorphous silicon nitride by a gas-phase reaction of silicon halide and ammonia.
2. Description of the Prior Arts
Sintered bodies of silicon nitride and silicon carbide have recently drawn attention from every field of technology as useful heat-resistant materials for various purposes. The materials are also widening their uses owing to the high corrosion-resistant property they possess.
Of these uses, the materials are regarded as particularly useful for gas turbine blades, radiant tubes in high temperature furnaces, and so forth owing to their exhibiting excellent property.
There have so far been proposed various methods for producing silicon nitride, of which the gas-phase reaction between silicon tetrachloride (SiCl.sub.4) and ammonia is the most popular method. This method is to subject silicon tetrachloride and ammonia, in required quantities, to the gas-phase reaction at a temperature range of from 590.degree. C. to 1500.degree. C. in the absence of oxygen to thereby synthesize .alpha.-crystal of silicon nitride in a single stage reaction, after which the synthesized product is immediately used for a raw material for the sintered body.
However, according to the studies made by the present inventors, it has been found out that this synthesizing method by the single stage reaction is low in its conversion rate of the reactants to silicon nitride in .alpha.-crystal which is considered favorable as the raw material for sintering, and moreover contains a considerable amount of amorphous silicon nitride mixed therein, which hinders the sintering property of the synthesized product.
In view of such problem inherent in the heretofore known method of synthesis of amorphous silicon nitride, the present inventors have found out that silicon nitride in .alpha.-crystal having favorable sintering property can be obtained at a high rate of yield by dividing the synthesizing reaction into two stages, wherein silicon tetrachloride and ammonia at a specified ratio are reacted at the first stage for a specified time and at a specified temperature to thereby positively synthesizing amorphous silicon nitride, and then said amorphous silicon nitride is maintained at the second stage in a current of nitrogen or ammonia for a specified time and at a specified temperature. On the basis of this finding, they have filed a patent application in Japan under Patent Application No. 53-41712.
According to this proposed method, powder of amorphous silicon nitride is produced in a body of a high temperature gas at the first stage reaction. The gas is hydrogen chloride as the principal constituent, in addition to which gaseous ammonium chloride is contained in a relatively large quantity. Such nitride powder is then separated and collected from the reaction product gas for the necessity of changing the reaction atmosphere for providing the same at the second stage reaction.
For such collecting expedient, there may be used various types of collecting devices, of which a bag filter type collector, an electric precipitator, and like other collectors are particularly favorable from the standpoints of device construction and collecting efficiency, in view of particle diameter of the produced powder being very fine.
Incidentally, most of the collectors which are generally used would be quite unsuitable because such collectors when exposed to a high temperature corrosive gas in the treatment system as in the present invention, would simply not withstand such conditions and would be undurable.
In such a case, it is a usual practice to cool the treatment system from outside of the device. However, when the external cooling is effected in this treatment system, ammonium chloride deposits and adheres onto the inner wall of the device with the consequent trouble of clogging the device interior.
In view of such point of problem, the present inventors have conducted various researches and studies for any expedient that can perform effective cooling, as the result of which they have found out that, when cool gas is directly mixed into the body of a high temperature gas containing therein amorphous silicon nitride powder and gaseous ammonium chloride, the ammonium chloride as deposited does not substantially adhere onto the inner wall of the device, and yet the system as a whole can be effectively cooled, whereby collection of amorphous silicon nitride powder can be done through a filter smoothly and over a long period of time.